It is previously known to use motor-powered chain saws or power saws for cleaving timber, e.g. for sawing boards and planks from round logs. Thus, SE 8703174-6, for instance, discloses a sawing device where the power saw is mounted on a carriage which is movable along a saw stand parallel to the log to be sawn.
There is however a need for simpler and thus less expensive devices or supplementary appliances which are easier to carry along and which can be readily mounted on the power saw to permit cleaving timber in flat and straight cuts.
Devices of the type stated by way of introduction exist in many different designs. Thus, for instance, supplementary appliances for power saws are available under the trade marks ALASKAN MK III and STIHL which have guide means cooperating with the guide bar and movable towards and away from the guide bar along guide members. Common to all these prior-art appliances is that the guide members are connected to the guide bar in two separate points, namely in a point immediately adjacent the power saw body and a point at the guide-bar tip. Between these two fixing points a free guide-bar length is delimited which becomes decisive of how thick logs can be sawn by means of the device. It goes without saying that such mounting of the device reduces the maximum available guide-bar length that could be used in sawing. Since the outer guide member is fixed to the tip of the guide bar, this must be so long that the tip projects by a sufficient margin on the rear side of the log. Thus, in the case of extremely thick timber dimensions or e.g. root swells, it is not possible to carry out the cleaving cut in two steps from each side of the log. In order, nevertheless, to obtain acceptable sawing widths, extremely long guide bars must be used. However, long guide bars suffer from the major drawback of requiring high driving power, which means larger and heavier power saws. These devices therefore tend to become unnecessarily heavy and unwieldy. Furthermore, it generally becomes necessary to provide holes in the guide bar in which the guide members can be screwed.
Similar devices are also known from e.g. U.S. Pat. Nos. 3,225,799, 4,244,104 and FR 2,298,418.
When using sawing appliances of the type mentioned by way of introduction, a further problem is encountered when the first saw cut is to be made in the log. In fact, this saw cut is decisive of the subsequent sawing result, since the sawing surface then formed serves as a guiding plane for the next saw cut. Some sort of a guiding board or the like has previously been fixed to the circumferential surface of the round log, for instance by nailing, and the first saw cut has been made immediately under the guiding board. It is of course very difficult to fix a guiding board in a straight and reliable manner to the round surface of a log. Moreover, the nails must not penetrate too deep into the log, since The saw chain could be damaged if encountering the nails. Further, it is mostly desirable to make the first saw cut in a plane parallel to the log axis. Since the logs are usually tapering towards the top end, the guiding board must be trestled at this end of the log to compensate for the taper. This of course complicates the sawing operation to a great extent.
In most cases, it is further desirable from a round log to produce square boards and planks, i.e. with four orthogonal boundary surfaces. This means that, in addition to a first flat or plane guiding cut, at least a second guiding cut must be made at right angles to the first. Hitherto, this has been done by turning the log 90.degree. and replacing the guiding board on the circumferential surface of the log and accurately positioning it thereon, after which a new saw cut is made. It goes without saying that this process of having the two guiding planes orthogonal to each other involves substantial difficulties. Although it is known in the art to employ a device, sold under the trade mark LUMBERMAKER, in the form of a supplementary appliance for power saws which permits sawing a flat or plane saw cut at right angles to a guiding board or a guiding plane, this known device is not capable of making parallel saw cuts. It is highly desirable, by means of one and the same device, in the form of a supplementary appliance for power saws, to be able to cleave a log both parallel and perpendicular to a guiding plane.
U.S. Pat. No. 3,134,409 discloses a combined device having on one side of the guide bar a first guide means to be applied and guided against a surface perpendicular to the guide bar, and on the other side of the guide bar a second guide means to be guided against a surface parallel to the guide bar. The second guide means is displaceable towards and away from the guide bar along a guide member and is connected to the guide member only at one end. However, this device is most unwieldy and, moreover, connected to the saw via the guide bar, more specifically in that the guide member is screwed substantially in the centre of the guide bar. Naturally, this highly restricts the usefulness and the performance of the saw, since the free guide-bar space available for sawing becomes very small. To compensate for this, the guide bar must be made longer, which in turn necessitates a larger and more powerful motor for driving the chain. This results in a heavy and most unwieldy construction consisting of the saw and the supplementary device.